A field-effect transistor controls electric conductivity of a semiconductor layer provided between a source electrode and a drain electrode, using voltage applied to a gate electrode. Basically, a field-effect transistor is a representative example of unipolar element in which either p-type carriers or n-type carriers (hole or electron) transport electric charge.
These field-effect transistors can form various switching elements or amplifying elements, depending on the combination; therefore the field-effect transistors are applied in various areas. For example, a switching element and the like for a pixel of an active matrix type display can be given as an application example.
As a semiconductor material used for a field-effect transistor, an inorganic semiconductor material typified by silicon has been widely used in the past. However, it is difficult to use a plastic substrate or a film for a substrate since a treatment at high temperature is necessary to form an inorganic semiconductor material as a semiconductor layer.
On the other hand, when an organic semiconductor material is used as a semiconductor layer, it is possible to form a film with relatively low temperature. Therefore, in principle, a field-effect transistor can be manufactured over a plastic substrate, etc. which does not offer resistance to high temperatures, as well as a glass substrate.
As described above, as an example of a field-effect transistor using an organic semiconductor material as a semiconductor layer (hereinafter, referred to as an “organic field-effect transistor”), a transistor using silicon dioxide (SiO2) as a gate insulating layer and pentacene as a semiconductor layer (refer to Non Patent Document 1) can be given. According to the report, the electron field-effect mobility is reported as 1 cm2/Vs, and it is also reported that the transistor performance which is equal to amorphous silicon can be obtained, even when an organic semiconductor material is used as a semiconductor layer.
By the way, in an organic field-effect transistor, carriers are transported between source and drain electrodes and a semiconductor layer, however the transistor characteristic, such as the electron field-effect mobility, is declined when the energy barrier exists at the interface between the source electrode and the semiconductor layer and between the drain electrode and the semiconductor layer. In order to improve it, it is proposed to use a lithium fluoride layer for the interface between the source electrode and the semiconductor layer and between the drain electrode and the semiconductor layer (refer to Patent Document 1), however, since it can be applied only for an n-channel type organic field-effect transistor, a kind of an organic semiconductor material is limited to the n-type. In addition, it is also proposed to dope the semiconductor layer with a conductivity imparting agent (refer to Patent Document 2), however, there is a problem that the conductivity imparting agent has low chemical stability. Furthermore, the adhesiveness between these electrode materials and organic semiconductor materials is also important in order to obtain a transistor having excellent resistance.
Thus, source and drain electrodes which can be used for an organic field-effect transistor using various organic semiconductor materials, which are chemically stable, and has good adhesiveness to the organic semiconductor material, are hoped for. It is because that an organic field-effect transistor having favorable electron field-effect mobility and excellent resistance can be obtained by applying such source and drain electrodes.
In addition, in an organic field-effect transistor, since source and drain electrodes are also used as wirings, high conductivity is required. However, source and drain electrodes having the above described characteristics and high conductivity have not been reported yet.
[Non Patent Document 1]
    Y. Y. Lin, D. J. Gundlach, S. F. Nelson, T. N. Jackson, IEEE Electron Device Letters, Vol. 18, 606-608 (1997)[Patent Document 1]    Japanese Patent Laid-Open No. 2003-298056[Patent Document 2]    Japanese Patent Laid-Open No. 2004-228371